The overall objective of this proposal is to characterize the process of differentiation of human monocytes-macrophages from the precursor cells in bone marrow and in the peripheral blood to the macrophages stage with particular emphasis on the variation of antigenic expression and on the development of specific functions. The experimental models used will include a) established lines of human hematopoietic cells and primary cultures of leukemia cells, induced to differentiate in vitro by different chemical or biological inducers and b) in vitro growth and differentiation of normal precursor and stem cells from bone marrow and peripheral blood. The general aims are to determine a chronological map of surface changes during normal monocyte-macrophage differentiation, to characterize the stage of differentiation at which lelukemic cells are arrested and to determine whether an imbalance in the expression of surface markers during induced differentiation of these cells exists.